Methods for uniform crimping and deployment of a polymer scaffold

ABSTRACT

A medical device-includes a scaffold crimped to a catheter having an expansion balloon. The scaffold is crimped to the balloon by a process that includes one or more balloon pressurization steps. The balloon pressurization steps are selected to enhance scaffold retention to the balloon while retaining, at least partially, the original balloon folds as the balloon is pressurized and de-pressurized within a crimper head. By at least partially retaining the original balloon folds, a uniformity of scaffold expansion by the balloon is improved.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to drug-eluting medical devices; moreparticularly, this invention relates to processes for crimping apolymeric scaffold to a delivery balloon.

2. Background of the Invention

The art recognizes a variety of factors that affect a polymericscaffold's ability to retain its structural integrity when subjected toexternal loadings, such as crimping and balloon expansion forces. Theseinteractions are complex and the mechanisms of action not fullyunderstood. According to the art, characteristics differentiating apolymeric, bio-absorbable scaffolding of the type expanded to a deployedstate by plastic deformation from a similarly functioning metal stentare many and significant. Indeed, several of the accepted analytic orempirical methods/models used to predict the behavior of metallic stentstend to be unreliable, if not inappropriate, as methods/models forreliably and consistently predicting the highly non-linear behavior of apolymeric load-bearing portion of a balloon-expandable scaffold(hereinafter “scaffold”). The models are not generally capable ofproviding an acceptable degree of certainty required for purposes ofimplanting the scaffold within a body, or predicting/anticipating theempirical data.

Moreover, it is recognized that the state of the art in medicaldevice-related balloon fabrication, e.g., non-compliant balloons forscaffold deployment and/or angioplasty, provide only limited informationabout how a polymeric material might behave when used to support a lumenwithin a living being via plastic deformation of a network of ringsinterconnected by struts. In short, methods devised to improvemechanical features of an inflated, thin-walled balloon structure, mostanalogous to mechanical properties of a pre-loaded membrane when theballoon is inflated and supporting a lumen, simply provides little, ifany insight into the behavior of a deployed scaffold. One difference,for example, is the propensity for fracture or cracks to develop in ascaffold. The art recognizes the mechanical problem as too different toprovide helpful insights, therefore, despite a shared similarity inclass of material. At best, the balloon fabrication art provides onlygeneral guidance for one seeking to improve characteristics of ascaffold.

Polymer material considered for use as a scaffold, e.g. PLLA or PLGA,may be described, through comparison with a metallic material used toform a scaffold, in some of the following ways. A suitable polymer has alow strength to weight ratio, which means more material is needed toprovide an equivalent mechanical property to that of a metal. Therefore,struts must be made thicker and wider to have the strength needed. Thescaffolding also tends to be brittle or have limited fracture toughness.The anisotropic and rate-dependant inelastic properties (i.e.,strength/stiffness of the material varies depending upon the rate atwhich the material is deformed) inherent in the material only compoundthis complexity in working with a polymer, particularly, bio-absorbablepolymer such as PLLA or PLGA.

Processing steps performed on, design changes made to a metal stent thathave not typically raised concerns for, or require careful attention tounanticipated changes in the average mechanical properties of thematerial, therefore, may not also apply to a scaffold due to thenon-linear and sometimes unpredictable nature of the mechanicalproperties of the polymer under a similar loading condition. It issometimes the case that one needs to undertake extensive validationbefore it even becomes possible to predict more generally whether aparticular condition is due to one factor or another—e.g., was a defectthe result of one or more steps of a fabrication process, or one or moresteps in a process that takes place after scaffold fabrication, e.g.,crimping. As a consequence, a change to a fabrication process,post-fabrication process or even relatively minor changes to a scaffoldpattern design must, generally speaking, be investigated more thoroughlythan if a metallic material were used instead of the polymer. Itfollows, therefore, that when choosing among different scaffold designsfor improvement thereof, there are far less inferences, theories, orsystematic methods of discovery available, as a tool for steering oneclear of unproductive paths, and towards more productive paths forimprovement, than when making changes in a metal stent.

It is recognized, therefore, that, whereas inferences previouslyaccepted in the art for stent validation or feasibility when anisotropic and ductile metallic material was used, such inferences wouldbe inappropriate for a scaffold. A change in a scaffold pattern mayeffect, not only the stiffness or lumen coverage of the scaffold in itsdeployed state supporting a lumen, but also the propensity for fracturesto develop when the scaffold is crimped or being deployed. This meansthat, in comparison to a metallic stent, there is generally noassumption that can be made as to whether a changed scaffold pattern maynot produce an adverse outcome, or require a significant change in aprocessing step (e.g., tube forming, laser cutting, crimping, etc.).Simply put, the highly favorable, inherent properties of a metal(generally invariant stress/strain properties with respect to the rateof deformation or the direction of loading, and the material's ductilenature), which simplify the stent fabrication process, allow forinferences to be more easily drawn between a changed stent patternand/or a processing step and the ability for the stent to be reliablymanufactured with the new pattern and without defects when implantedwithin a living being.

A change in the pattern of the struts and rings of a scaffold that isplastically deformed, both when crimped to, and when later deployed by aballoon, unfortunately, is not as easy to predict as a metal stent.Indeed, it is recognized that unexpected problems may arise in scaffoldfabrication steps as a result of a changed pattern that would not havenecessitated any changes if the pattern was instead formed from a metaltube. In contrast to changes in a metallic stent pattern, a change in ascaffold pattern may necessitate other modifications in fabricationsteps or post-fabrication processing, such as crimping andsterilization.

Scaffolds used to treat coronary vessels experience, for the most part,a primarily radial loading. However, scaffolds intended for peripheralvessels experience a quite different loading, to such an extent that thetraditional measure of a stent's fitness for use, i.e., its radialstrength/stiffness, is not an accurate measure of whether the scaffoldwill have sufficient strength to support the peripheral vessel. This isbecause a peripheral scaffold is placed in a significantly differentenvironment from a coronary scaffold. The vessel size is larger. Andthere is much more movement of the vessel, especially when located closeto an appendage. As such, a scaffold intended for a peripheral vesselwill need to be able to sustain more complex loading, including acombination of axial, bending, torsional and radial loading. See e.g.Bosiers, M. and Schwartz, L., Development of Bioresorbable Scaffolds forthe Superficial Femoral Artery, SFA: CONTEMPORARY ENDOVASCULARMANAGEMENT (‘Interventions in the SFA” section). These and relatedchallenges facing peripherally implanted stents and scaffolds are alsodiscussed in U.S. application Ser. No. 13/015,474.

One challenge, in particular, facing a peripheral scaffold is crimpingto a balloon and expansion of the scaffold when the balloon is inflated.Problems arise where, on the one hand, the scaffold cannot be crimped tothe desired size without introducing structural failure, i.e., fracture,or excessive cracking, either in the crimped state or when expanded fromthe crimped state by a balloon. On the other hand, a scaffold can becrimped and deployed, yet deploys with non-uniformity in its deployedstate. In these cases the scaffold is susceptible to acute or fatiguefailure as the irregularly-deployed rings and/or cells, loaded beyondtheir design limits as a consequence of the non-uniform deployment, havea reduced acute or fatigue life within the vessel.

A film-headed crimper has been used to crimp polymer scaffolds toballoons. Referring to FIG. 8A, there is shown a perspective view of acrimping assembly 20 that includes three rolls 123, 124, 125 used toposition a clean sheet of non-stick material between the crimping bladesand a metal stent prior to crimping. For example, upper roll 125 holdsthe sheet secured to a backing sheet. The sheet is drawn from thebacking sheet by a rotating mechanism (not shown) within the crimperhead 20. A second sheet is dispensed from the mid roll 124. Aftercrimping, the first and second (used) sheets are collected by the lowerroll 123. As an alternative to rollers dispensing a non-stick sheet,each metal stent may be covered in a thin, compliant protective sheathbefore crimping.

FIG. 8B illustrates the positioning the first sheet 125 a and secondsheet 124 a relative to the wedges 22 and a metal stent 100 within theaperture of the crimping assembly 20. As illustrated each of the twosheets are passed between two blades 22 on opposite sides of the stent100 and a tension T1 and T2 applied to gather up excess sheet materialas the iris of the crimping assembly is reduced in size via theconverging blades 22.

The dispensed sheets of non-stick material (or protective sheath) areused to avoid buildup of coating material on the crimper blades forstents coated with a therapeutic agent. The sheets 125 a, 124 a arereplaced by a new sheet after each crimping sequence. By advancing aclean sheet after each crimp, accumulation of contaminating coatingmaterial from previously crimped stents is avoided. By using replaceablesheets, stents having different drug coatings can be crimped using thesame crimping assembly without risk of contamination or buildup ofcoating material from prior stent crimping.

In light of the foregoing problems, there is a need to improve theuniformity of deployment for a peripherally-implanted scaffold, whilemaintaining an appropriate balance among a desired retention force andminimal crossing profile for delivery to a target site. And there is acontinuing need to address structural integrity of aperipherally-implanted scaffold after repeated axial, bending and radialloading characteristic of a peripheral vessel.

SUMMARY OF THE INVENTION

The invention provides methods for increasing uniformity of scaffoldexpansion via a balloon inflated delivery system while maintaining adesired balloon-scaffold retention force to prevent dislodgment of thescaffold from the balloon during delivery of the scaffold to a targetlocation in a vessel.

It has been demonstrated that the retention force of a crimped polymerscaffold on a delivery balloon may be increased by a crimping processthat includes crimping the scaffold to the balloon while the balloon ispressurized; that is, the balloon is pressurized at the same time as thescaffold's outer diameter is being reduced by crimper blades. Additionalfeatures of such a crimping process include heating the scaffold to atemperature close to, but below the glass transition temperature (TG) ofthe polymer material and applying balloon pressure during dwell periods(i.e., balloon pressure is applied when the scaffold diameter is heldconstant).

However, when these same processes are applied to aperipherally-implanted scaffold having a relatively large diameterreduction when crimped to a balloon, e.g., 6:1 ratio of crimped toexpanded diameter, problems were encountered upon expansion of thescaffold in vivo. The scaffold did not consistently expand in a uniformmanner. As a consequence, ring struts and/or cell structures, whichprovide radial strength and stiffness for the scaffold, inherit anun-even distribution of stresses and strains. Over-expanded cells arecalled upon to sustain higher-than-normal stresses and strains whileneighboring under-expanded cells are underutilized. The balloon-inducedstresses and strains associated with over-expanded cells can exceed thematerial's ultimate stress and strain level at deployment, which resultin crack formation or fracture, or exhibit a reduced fatigue life orfracture toughness, in which case fracture can occur immediately, aftera few days, or after weeks of implantation. Explants of scaffolds fromanimal studies have shown this type of behavior for scaffolds expandedin a non-uniform manner.

Strut fractures in peripherally implanted scaffolds are a re-occurringproblem. As compared to coronary scaffolds, the causes are not fullyunderstood, but are believed to reside in the combination of the largediameter reduction/expansion for the scaffold and the complex loadingenvironment of a peripherally-implanted scaffold (as compared to acoronary scaffold). In view of these problems, it is therefore desirableto arrive at a more uniform expansion for a peripherally-implantedscaffold.

In view of these needs, a crimping process is discovered that maintainsa desired minimum scaffold retention on the balloon and crossingprofile, while significantly improving uniformity of expansion and thenumber of intact ring structures, or rings that are devoid of fracturedstruts.

According to one aspect of the invention, a crimping process is devisedthat increases the uniformity of scaffold cell expansion by pressurizingthe balloon at an adequate stage during the crimping process to maintaina more uniform folded balloon morphology. That is, balloonpressurization occurs once the scaffold has attained an intermediatecrimped diameter, which causes the balloon to at least partially retainthe original folds in the balloon when the pressurization is relievedand the scaffold crimped to its final diameter.

According to another aspect of the invention, based on conducted studiesin vivo and in vitro, it has been found that there exists a criticalcrimp outer diameter (CCOD) for a scaffold expanded by a folded orpleated balloon, which identifies a maximum diameter for balloonexpansion. By computation of the CCOD for a given scaffold and balloon,one can estimate the maximum OD for the scaffold for initiating balloonpressurization to achieve a good stent retention force to the balloonwhile also retaining uniformity of expansion. Two methods for computingthe CCOD, one more conservative than the other, were derived based onstudies. The CCOD may be used to estimate the maximum outer diameter ofthe scaffold before the start of balloon pressurization, to ensureuniform scaffold expansion.

The process may include several crimping steps. Following each crimpingstep, a dwell period occurs to allow the scaffold material to relievebuilt up strain before further reducing its diameter. After one or moreinitial crimping steps, the partially-crimped scaffold is removed fromthe crimper head to check alignment on the balloon. This step isreferred to as the final alignment, or check final alignment step in thedisclosure. After checking alignment, the scaffold is returned to thecrimper to perform the final crimp. The final diameter reduction isperformed while the balloon is pressurized to urge balloon materialbetween gaps in the scaffold struts. In contrast to previous crimpingmethods, only this single pressurization step is used, according to oneembodiment.

According to another aspect of the invention, a crimping process isdisclosed that improves the uniformity of a crush-recoverable scaffoldexpansion within a peripheral vessel.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in the presentspecification are herein incorporated by reference to the same extent asif each individual publication or patent application was specificallyand individually indicated to be incorporated by reference. To theextent there are any inconsistent usages of words and/or phrases betweenan incorporated publication or patent and the present specification,these words and/or phrases will have a meaning that is consistent withthe manner in which they are used in the present specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a planar view of a polymer scaffold exhibiting non-uniformexpansion of its closed cells for providing radial strength andstiffness to the implanted scaffold.

FIG. 2 is a front cross-sectional view of a balloon catheter after theballoon is pressurized within the bore of a scaffold having a first,partially crimped diameter.

FIG. 3 is a front cross-sectional view of a balloon catheter after theballoon is pressurized within the bore of a scaffold having a second,partially crimped diameter.

FIG. 4 is a drawing detailing aspects of the scaffold depicted in FIG. 1prior to being crimped to a balloon, or in its as-lased (pre-crimp)configuration.

FIG. 5 is a partial perspective view of a portion of the scaffold ofFIG. 4.

FIG. 6 is a drawing of the cell enclosed within phantom box VB of FIG.4.

FIG. 7 is a table providing dimensions for the scaffold of FIG. 4.

FIG. 8A is a perspective view of a film-headed crimper that was used tocrimp the scaffold of FIG. 1.

FIG. 8B is a frontal view of the head of the film-headed crimper ascrimper jaws are being brought down on a scaffold.

FIG. 9A is a sketch of a five-fold balloon in the unexpanded state.

FIG. 9B is a diagram of the balloon of FIG. 9A under a partial inflationpressure causing one folds to open.

DETAILED DESCRIPTION OF EMBODIMENTS

A scaffold crimped and expanded by a balloon according to the disclosureis formed from a tube made by extruded PLLA. The tube forming processdescribed in US Pub. No. 2010/0025894 may be used to form this tube. Thefinished, solidified polymeric tube of PLLA may then be deformed inradial and axial directions by a blow molding process whereindeformation occurs progressively at a predetermined longitudinal speedalong the longitudinal axis of the tube. For example, blow molding canbe performed as described in U.S. Publication No. 2009/0001633. Thisbiaxial deformation, after the tube is formed, can produce noticeableimprovement in the mechanical properties of the scaffold structuralmembers cut from the tube without this expansion. The degree of radialexpansion that the polymer tube undergoes characterizes the degree ofinduced circumferential molecular or crystal orientation. In a preferredembodiment, the radial expansion ratio or RE ratio is about 450% of thestarting tube's inner diameter and the axial expansion ratio or AE ratiois about 150% of the starting tube's length. The ratios RA and AE aredefined in US Pub. No. 2010/0025894.

The above scaffold's outer diameter may be designated by where it isexpected to be used, e.g., a specific location or area in the body. Theouter diameter, however, is usually only an approximation of what willbe needed during the procedure. For instance, there may be extensivecalcification that breaks down once a therapeutic agent takes effect,which can cause the scaffold to dislodge in the vessel. Further, since avessel wall cannot be assumed as circular in cross-section, and itsactual size only an approximation, a physician can choose to over-extendthe scaffold to ensure it stays in place. For this reason, it ispreferred to use a tube with a diameter larger than the expecteddeployed diameter of the scaffold.

As explained in greater detail below and in U.S. application Ser. No.13/015,474, the “'474” application, and in particular the “V59” scaffoldof FIGS. 5B and 6B) a scaffold has a 8 mm as lased diameter, a finalcrimp outer diameter of about 2.3 mm (prior to removing the crimper jawsfrom the scaffold) which is less than a “theoretical minimum diameter”for the scaffold pattern, an inflated diameter of about 6.5-7.0 mm (6.5mm average vessel size) and maximum expanded diameter by a post-dilationcatheter balloon of about 9.5 mm (unless stated otherwise, scaffolddiameter shall refer to the scaffold outer diameter). The diameter afterremoval from the crimper is about 0.092 in.

According to one embodiment, a scaffold crimped in accordance with theinvention may have a ratio of inflated to crimp diameter of betweenabout 2.5:1 and 3:1, for a 6.0 mm nominal balloon diameter, and a ratioof pre-crimp to crimped diameter of about 3:1 to 3.5:1 or about 4:1which ratio generally depends on the inflated diameter, crossing profileand/or vessel diameter. More generally, with respect to a vesseldiameter (VD), Equations 1 and 2 of the '474 application may be used todetermine SD_(PC) and SD_(I) for a scaffold possessing desirableproperties for implantation in peripheral vessels, which equations areconsidered part of the disclosure. Equations 1 and 2 are reproducedbelow.

A scaffold has a pre-crimp diameter (SD_(PC)) meaning the diameter ofthe scaffold before it is crimped to its delivery balloon, and aninflated diameter (SD_(I)). The scaffold is crimped to theballoon-catheter and intended for delivery to a vessel within the body.The average vessel diameter where the scaffold is to be implanted is VD.SD_(I) is about 1.2 times greater than VD. For purposes of thedisclosure, VD can range from about 5 mm to 10 mm and SD_(PC) can rangebetween about 6 to 12 mm. According to another aspect of invention:1.1×(VD)≦SD _(PC)≦1.7×(VD)  EQ. 11.1×(SD _(I))×(1.2)⁻¹ ≦SD _(PC)≦1.7×(SD _(I))×(1.2)⁻¹  EQ. 2

There may be a greater need to modify a crimp process to increase theretention force of a coronary scaffold while on a balloon, especiallyfor coronary scaffolds having short lengths, as compared to aperipherally implanted scaffold. A “retention force” for a scaffoldcrimped to a balloon means the maximum force, applied to the scaffoldalong the direction of travel through a vessel that the scaffold-balloonis able to resist before dislodging the scaffold from the balloon. Theretention force for a scaffold on a balloon is set by a crimpingprocess, whereby the scaffold is plastically deformed onto the balloonsurface to form a fit that resists dislodgment of the scaffold from theballoon. Factors affecting the retention of a scaffold on a balloon aremany. They include the extent of surface-to-surface contact between theballoon and scaffold, the coefficient of friction of the balloon andscaffold surfaces, and the degree of protrusion or extension of balloonmaterial between struts of the scaffold. As such, the pull off orretention force for a scaffold generally varies with its length.Therefore the shorter the scaffold the more likely it can becomedislodged when the catheter is pushed through tortuous anatomy. Aperipheral scaffold, however, is typically much longer than a coronaryscaffold. The retention force is therefore more often not as much of aconcern as in the case of a short-length coronary scaffold.

With this said, however, it is still more of a challenge to secure aperipherally-implanted scaffold to a balloon than in the case of anequivalent metal stent to achieve the same amount of retention force tothe balloon and without damaging the scaffold. This is because of thelimited temperature range available for crimping a scaffold, e.g.,between 5 to 15 degrees below the low end of the glass transitiontemperature (or “TG-low”) for a scaffold in a preferred embodiment,verses a metal stent and the generally more brittle properties of avessel supporting polymer material. Also, given the reduced strength andstiffness properties, struts of a polymer scaffold must be thicker forthe equivalent properties of a metal strut, which results in reducedspace available for balloon material to lodge between scaffold struts.

TABLE 1 summarizes the crimping stages or steps that were used to crimpthe scaffold depicted in FIGS. 4-6 to a balloon catheter. As will beappreciated, the crimping process is time consuming given in thevisco-elastic properties of the polymer material in the scaffold(preferably PLLA) and extreme diameter reduction (about 6:1 required toachieve the target crossing profile while retaining a pre-crimp diameterthat exceeds the nominal and post-dilated inflated diameters). Ninestages, or steps were programmed into the crimp mechanism as the controlsettings to crimp the scaffold. The crimp mechanism used was thefilm-headed crimper illustrated in FIGS. 8A and 8B.

TABLE 1 Control settings for crimping process Crimp Crimp head BalloonControl Diameter Speed Dwell Balloon pressurization settings (in.)(in/sec) times pressurization dwell times Initial 0.640 point Stage 10.354 0.300 0 Amb 0 Stage 2 0.270 0.005 30 Amb 0 Stage 3 0.210 0.005 30Amb 0 Stage 4 0.160 0.005 30 Amb 0 Stage 5 0.130 0.005 30 Pressure 30Stage 6 0.140 0.050 30 Amb 0 Stage 7 0.130 0.005 30 Pressure 30 Stage 80.100 0.005 30 Pressure 30 Stage 9 0.062 0.005 30 Amb 170

The crimp temperature was approximately 48 degrees Celsius and thematerial used for the scaffold was PLLA. Column 2 provides the diameterof the crimper jaws at each stage, with the diameter 0.354 incorresponding to the pre-crimp diameter of the scaffold. The final crimpdiameter setting is 0.062 in. When removed the crimper, the scaffoldrecoils to about 0.092 in. Column 3 shows the rate at which the crimperjaws are reduced. Thus, between Stage 1 and 2 the crimper diameter isreduced at a rate of 0.3 in/sec. Following each diameter reduction, thecrimper dwells for 30 seconds (column 3), which gives the visco-elasticmaterial the time needed to relieve stresses before the scaffold isfurther deformed by crimper blades.

Three pressurization stages occur during the crimping process, with theinitial pressurization for 30 seconds occurring with a diameter of 0.13in (Stage 5). Following the initial pressure stage, the crimper opens toallow the scaffold to be removed from the crimper to check its alignmenton the balloon. The scaffold is then placed back into the crimper andthe jaws are re-set to 0.14 in (Stage 6). An intermediate balloonpressure of 50 psi is applied after the crimper jaws reach 0.13 in(Stage 7), i.e., during the 30 second dwell at 0.13 in. Once the crimperjaws reach 0.10 in. (Stage 8—final pressure step) balloon pressure isapplied and maintained at about 50 psi. After a 30 second dwell at 0.10,the balloon pressure is relieved and the jaws are set to a diameter of0.062 in (Stage 9). A 170 second dwell period at about ambient pressureis initiated to relieve strain in the scaffold, which helps to reducerecoil after the scaffold is removed from the crimper. A constrainingsheath is then placed over the scaffold immediately after removing itfrom the crimper, to limit recoil of the scaffold.

The scaffold described in FIGS. 4-6, when crimped according to the TABLE1, was able to achieve a crimped diameter below the theoretical minimumdiameter (as defined in the '474 application), and exhibited nosignificant or re-occurring signs of fracture or loss of strength whenexpanded in bench tests or during in-vitro accelerated life testingand/or fatigue testing. When the scaffold was deployed to supportvessels in healthy porcine models, however, several cracks and/orfractures developed and a non-uniform expansion of the scaffold wasobserved.

FIG. 1 illustrates an example of non-uniform deployment behaviorexhibited by the scaffold of FIG. 4 when crimped according to theprocess of TABLE 1. This drawing is based on a FINESCAN image of anexpanded scaffold. The region A of the scaffold in FIG. 1 shows regionsof over-expanded cells 204 (FIG. 4), e.g., regions A1, A2, that havebeen over-expanded. As a result, the crown angles at A1, A2 areincreased beyond their design angles, which induces high local stressesnear crowns. The region B shows the corresponding cells 204, e.g., B1,B2 that are under-expanded. Hence, the angles at these crowns are lessthan intended when the scaffold attains its expanded diameter. While thenet result is the intended expanded diameter, e.g., between about 6-7 mmfor a an average 6.0 mm diameter vessel, the distribution of stresses inthe cells 204 is uneven and affects the structural integrity of thescaffold.

While the areas of high stress in region A are in large sustainable whenthe scaffold is initially expanded within the vessel, the animal studieshave shown that after repeated loading cracks develop due to reducedfatigue toughness at the crowns. The same behavior was not seen duringthe in-vitro or bench testing. This result lends further support to theview that fracture propensity is especially acute, and complex, when ascaffold is supporting a peripheral vessel. As mentioned earlier, aperipheral scaffold, in contrast to a coronary scaffold, is subjected toa combined axial, bending and radial loading, as opposed to primarily aradially loading. This complex loading environment is believed to be achief cause for the observed fracture problems. For instance, it isbelieved that the axial contraction, and expansion of a peripheralvessel is a significant contributing factor to the fatigue failureobserved during the course of the animal studies.

One feature of the scaffold of FIG. 7 that enables it to achieve a 6:1ratio of pre-crimp diameter to crimped diameter is its zero-radius atthe crown, as defined in the '474 application. The zero-radius crownenables the scaffold to be crimped down to, and even exceed itstheoretical minimum crimped diameter without fracture when crimped orexpanded from the crimped diameter. However, it is suspected that when acrown angle for this scaffold is exceeded, or nearly exceeded thepre-crimp crown angle, which can be thought of as a maximum design anglefor radial strength and stiffness when the scaffold is being loaded bythe vessel, the scaffold becomes susceptible to fracture or crackpropagation at the crown, which can severely reduce radial stiffness andstrength for the scaffold.

In more general terms for polymer scaffold, including those havinglarger crown radii than the V59 scaffold described in FIG. 7, anon-uniform expansion, which causes some crown angles to exceed theintended crown angle, increases the chances that the pre-crimp anglewill be exceeded when the scaffold is loaded by the vessel, since wheninitially expanded the crown has already exceeded the intended crownangle. As a consequence, the scaffold develops a higher propensity forfatigue failure in region A of the scaffold because this is where crownangles are higher than intended. Vessel dynamics will likely increasethese angles even further. It is therefore desirable to arrive at acrimping process that avoids excessive crown angles, e.g., anglesextending between struts that exceed, or even approach the angle formedwhen the scaffold was cut from the polymer tube, when the crimpedscaffold is expanded by the balloon. This need is particularly importantwhen a small radii at the crown is used, such as a zero-radius crown aswas used in the V59 scaffold described in FIG. 7.

Referring again to TABLE 1, the balloon is inflated to 50 psi at threestages of the process: after the scaffold diameter is reduced from 0.16in to 0.13 in and prior to final alignment (post-Stage 5), after thediameter is reduced from 0.14 in to 0.13 in (Stage 7) and again whilethe diameter is reduced from 0.13 in to 0.10 in (Stage 8). As explainedin more detail in U.S. application Ser. No. 13/089,225, the balloon maybe inflated to increase the retention force between the scaffold andballoon. By inflating the balloon at larger diameters, e.g., when thescaffold has a 0.13 in diameter, there is more space available betweenthe scaffold struts for balloon material to extend (sometimes known as“balloon puffing”). When balloon material is disposed between thestruts, the retention force of the scaffold on the balloon increases.Additionally, it is believed that by applying balloon pressure after adiameter reduction any developing irregular deformation of the strutscan be compensated-for by a counteracting balloon pressure applied tothe irregular crimped struts. Causes for irregular crimping areexplained in more detail in U.S. application Ser. No. 12/861,719.Accordingly, for some scaffold embodiments without this balloon pressureapplied the scaffold can be susceptible to irregular crimping, which canresult in high stress areas in the crowns, cracking or flipping ofstruts. For example, it was observed that the scaffold pattern depictedin FIG. 4 of the '474 application was susceptible to irregular crimpingand even flipping of struts, which could be compensated for by using aballoon to support the scaffold when it was crimped, especially duringthe initial stages of the crimping process. The scaffold of FIG. 4,however, did not exhibit the same problems during crimping. However, invivo studies revealed a non-uniform expansion behavior for the scaffold.

FIG. 2 shows a cross-section of the balloon catheter 2 with scaffoldremoved. This view was obtained after the scaffold had attained acrimped diameter of 0.13 in diameter and the balloon 6 was inflated to50 psi (the scaffold was crimped to 0.13 in, the balloon 6 inflated, thescaffold and catheter 2 removed from the crimper, the scaffold removedfrom the balloon 6, then the catheter shaft 4 was cut about midway toshow the cross-section of the balloon 6). As can be seen, the folds 8 ofthe balloon 6 are distributed asymmetrically or non-uniformly about theshaft 4. The right-hand side folds 8 and left-hand side folds areirregular, such that the original folds in the balloon essentially nolonger exist. The area B′ folds are compressed, or lay flat on thecatheter shaft, while the folds seem to accumulate or build by in areaA′. This suggests that when the scaffold was later crimped to theballoon in this state, the scaffold WAS either irregularly crimped dueto an uneven balloon surface receiving the scaffold, or non-uniformballoon forces acted on the scaffold when the balloon is pressurized toexpand the scaffold, or a combination of these effects. When comparingthe over-expanded cells 204 in region A to the accumulated folds in areaA′, it was concluded that the accumulated balloon material on the righthand side of FIG. 2 caused the over-expanded cells.

It was also contemplated that the sheets of the film-headed crimper,which impart a torque or twisting on the scaffold during the crimpingprocess, might have also contributed to the arrangement of the balloonfolds in FIG. 2. When the balloon is allowed to expand at the 0.13 inchdiameter in the Table 1 process, it was believed that perhaps a twistingon the scaffold by the polymer sheets may have contributed to the unevenballoon folds illustrated in FIG. 2. However, it was found that thepolymer sheets were not a significant contributing factor based on acomparison of expanded scaffold with and without using a film-headedcrimper

A modified crimp process according to the disclosure increased theuniformity of cell expansion over the scaffold length; while notablyalso not unacceptably reducing the retention force between scaffold andballoon, requiring a reduction in the desired ratio of inflated to crimpdiameter or pre-crimp to crimp diameter, or a re-design of the scaffoldstructure. For example, in the case of the V59 scaffold an acceptablescaffold-balloon retention force was retained, the scaffold design wasunaltered, e.g., the scaffold still retained its zero-radius crowns, andthe same 6:1 ratio of pre-crimp to crimp diameter ratio when using themodified process (hence, a low crossing profile was retained).Additionally, in vivo studies tests revealed a significant reduction inthe number of fractures in struts of the scaffold as compared to thesame scaffold using the process of TABLE 1.

The process used to crimp the scaffold for the in vivo studies issummarized in TABLE 2, below. As compared to the process in TABLE 1,balloon pressure is applied only during the final pressure step, i.e.,when the scaffold diameter is reduced from 0.1 in to 0.062 in by thecrimper. Prior to this step the balloon was not pressurized.

TABLE 2 Control settings for modified crimping process Outer CrimpBalloon Crimp Diameter head Dwell Balloon pressurization Control settingSpeed times pressurization dwell times settings (in.) (in/sec) (sec) (50psi) (sec) Initial 0.640 point Stage 1 0.354 0.300 0 Amb 0 Stage 2 0.2700.005 30 Amb 0 Stage 3 0.210 0.005 30 Amb 0 Stage 4 0.160 0.005 30 Amb 0Stage 5 0.130 0.005 30 Amb 0 Stage 6 0.140 0.050 30 Amb 0 Stage 7 0.1300.005 30 Amb 0 Stage 8 0.100 0.005 30 Pressure 30 Stage 9 0.062 0.005 30Amb 170

FIG. 3 shows the cross-section of the balloon 6 when the scaffold hadattained a 0.10 in diameter using the Table 2 process. As can beappreciated from the drawing, the original balloon folding still exists;i.e., the folds are more evenly distributed about the catheter shaft 4and retain most of their originally folded directions, as indicated bythe arrows 11. Similarly, the compliance of the balloon surface is moreuniform about the circumference in FIG. 3 verses FIG. 2, whichcontributes to a more consistent crimp of ring struts about thecircumference; hence, a more uniform expansion of the scaffold than inthe case of FIG. 2.

Comparing the diameter of the scaffold to when balloon pressure isapplied in TABLE 2, a substantial improvement in uniformity of expansionwas discovered when balloon pressure was applied only after the scaffoldhad been crimped to about 30% of its pre-crimp diameter. It will beunderstood that 30% is an approximation of the maximum diameter thatwill substantially improve the uniformity of scaffold expansion. Forexample, it is expected that a diameter that is 32%, or 33% can alsoproduce a noticeable improvement.

As mentioned earlier, in vitro and in vivo (explants) studies ofscaffold performance using the crimping process of TABLES 1 and 2 FIG. 1and modified crimping process were conducted using the V59 scaffolddescribed in FIGS. 4-7. These tests compared the expanded scaffoldsshapes to inspect the uniformity of expansion, as well as the number ofcracked or fractured struts rings between the two processes. The testsalso compared the dislodgment or scaffold-to-balloon retention forceusing the two crimping processes. Healthy porcine iliofemoral arteryexplants were obtained, which provided the expanded scaffold within theartery of the porcine model. Inspection of these explants wasfacilitated using FINESCAN imaging.

Dislodgment or retention forces were tested by applying a tape to thesurface of the crimped scaffold then measuring the force required todislodge the scaffold from the balloon by pulling upon the tape. Thetests revealed that the dislodgment force was reduced (by about ½) whenusing the modified process. However, this retention force, which wasmeasured at about 1 lbf, is believed high enough to safely deliver thescaffold to the target location in the vessel without risk of thescaffold becoming dislodged from the balloon.

Table 3 shows a comparison of the V59 scaffolds when expanded using thetwo crimping processes. The scaffold had a nominal expanded diameter ofabout 6.5 mm and a post-dilation diameter of about 7.0 mm. The valuesgiven are mean plus standard deviation.

TABLE 3 comparison of expanded V59 scaffold properties Table 1 Table 2process process used to used to crimp V59 crimp Testing scaffold V59scaffold method Percent of intact rings (i.e., no  75 +/− 7% 100 +/− 2%In-vivo visible fractures in rings) at 6.5 mm expanded diameter. Numberof fractures at 10.5 mm 7.2 +/− .1  1.7 +/− .4 In-vitro diameter(maximum expanded diameter for balloon)

As can be appreciated from these results, when the modified crimpingprocess is used there is a dramatic increase in the number of intactrings (mean of 75% vs. 100%), and the number of fractures at 10.5 mm isreduced significantly (mean of 7.2 vs. 1.7).

Based on the foregoing findings, it was concluded that the originalfolds in the balloon can be maintained or substantially maintained,which leads to a significant improvement in the uniformity of expansionand increase in the number of intact struts, without adversely affectingother important crimping objectives, as explained earlier. Moreover,based on these observations, including results of the in vivo studies,valuable insight was gained as to the appropriate control settings for acrimping process in the more general case of a scaffold crimped to afolded balloon.

To achieve more uniform scaffold open cells and strut angles onexpansion, a critical crimp OD may be defined to maximize both theuniformity of scaffold expansion and scaffold dislodgement force. Thiscritical crimp OD is the maximum crimp diameter to initiate balloonpressurization above which the expansion would become non-uniform. Thiscritical crimp diameter would allow for the best combination of scaffoldretention (a sooner pressurization is better at greater OD) anduniformity of scaffold cells and strut angles on expansion (laterpressurization is better at smaller OD).

According to a method 1 of estimating a critical crimp OD (CCOD), onestarts with the condition of a single balloon fold opening, while theother folds remain substantially folded. FIGS. 9A and 9B show a deflatedand partially inflated 5-fold balloon, respectively. In FIG. 9A each ofthe folds Y1, Y2, Y3, Y4 and Y5 are arranged in their deflatedconfiguration (prior to any balloon pressurization during crimping).FIG. 9B shows the balloon partially inflated with the balloon portions(Y1A, Y1B, Y1C) that formed fold Y1 completely opened up so that theoriginal fold from FIG. 9A is lost. Upon balloon expansion from thestate in FIG. 9B the scaffold portion above Y1A, Y1B, Y1C begins toexpand at a different rate than the scaffold portions above foldedregions Y2, Y3, Y4, and Y5 (NOTE: FIG. 9B is not intended to show anactual configuration of the balloon, but rather serves only as an aideto better appreciate the approach taken under method 1).

Equations 1, 2, 3 (below) derive the CCOD under method 1. The arc lengthof (n−1) folds of an n-fold balloon isArc-length for (n−1) folds=πØ₁((n−1)/n)  Eq. 1

Where Ø₁ the outer diameter of the deflated balloon as pressed onto theguidewire lumen 4. For the five-fold balloon of FIG. 9A, n=5 andØ₁=0.072 in (1.8288 mm). Eq. 1 therefore yields 4.596 mm for the summedarc lengths of the folded portions Y2, Y3, Y4 and Y5. Next, the arclength of the single expanded (or completely unfolded) fold, e.g., thesums of Y1A, Y1B, Y1C balloon fold portions in FIG. 9B, is found fromEq. 2, belowArc-length for single fold (inflated)=(πØ₂)/n  Eq. 2

Where Ø₂ is the outer diameter of the n-fold balloon when nominallyinflated. For the five-fold balloon of FIG. 9A Ø₂=6 mm (nominal ballooninflation) and Eq. 2 therefore yields 3.7699 mm, which is an arc lengthof a single fold that is unfolded as it would be at the nominal balloondiameter of 6 mm.

The maximum balloon diameter for single unfolded fold is the sums ofEqs. 1 and 2 divided by π. From this relation the CCOD for the scaffoldis found by adding 2-times the scaffold wall thickness (t), which leadsto Eq. 3.CCOD(method 1)=Ø₁((n−1)/n)+(Ø₂)/n+2t  Eq. 3

For the V59 scaffold Eq. 3 yields 0.126 in, which is found to be a verygood approximation of the maximum scaffold diameter size that can bepresent at the onset of balloon pressurization during crimping, based onthe conducted tests, without causing a non-uniform expansion. Accordingto a preferred embodiment, a 0.100 in control setting is chosen;however, a larger diameter may be used without causing non-uniformexpansion.

Under a method 2 CCOD is expressed in Eq. 4, below. Here the CCOD, maybe defined as a function of the length of folds or pleats, or LF, theouter diameter of the catheter's guidewire wire, OD_(IM), and thescaffold wall thickness (t), as follows:CCOD(method 2)=2(Ø₂/2n+t)+OD_(IM)  Eq. 4

Eq. 4 calculates the CCOD from the condition of two open folds directlyacross from each other, whereas Eq. 3 calculates the CCOD based on asingle open fold. For example, the CCOD (method 2) for the V59 scaffoldusing a 6 mm nominal balloon OD (5-fold balloon catheter) is computed asfollows:Ø₂/2n=6 mm/(2×5 folds)=0.60 mmT _(strut)=0.28 mmOD_(IM)=1.05 mmCCOD=2×(0.60 mm+0.28 mm)+1.05 mm=2.81 mm (0.11 in)

Method 2 may be considered a more conservative estimate of CCOD.

Eqs. 3 and 4 are valid at relatively low inflation pressures that merelyunravel the balloon such as a few atmospheres of pressure. Also,application of vacuum pressure can help refold the balloon, which shouldincrease the CCOD for uniform expansion. However, one must also considerthe need to pillow the balloon ends to help aid in scaffold retention(as such, it may not be desirable to apply vacuum pressure to helpre-fold the balloon folds).

The properties of a scaffold crimped according to the disclosure willnow be described with reference to FIGS. 4-7. Additional aspects of thisscaffold are described in U.S. application Ser. No. 13/015,474.

Referring to FIG. 4, the scaffold pattern 200 includeslongitudinally-spaced rings 212 formed by struts 230. A ring 212 isconnected to an adjacent ring by several links 234, each of whichextends parallel to axis A-A. In this first embodiment of a scaffoldpattern (pattern 200) four links 234 connect the interior ring 212,which refers to a ring having a ring to its left and right in FIG. 4, toeach of the two adjacent rings. Thus, ring 212 b is connected by fourlinks 234 to ring 212 c and four links 234 to ring 212 a. Ring 212 d isan end ring connected to only the ring to its left in FIG. 4.

A ring 212 is formed by struts 230 connected at crowns 207, 209 and 210.A link 234 is joined with struts 230 at a crown 209 (W-crown) and at acrown 210 (Y-crown). A crown 207 (free-crown) does not have a link 234connected to it. Preferably the struts 230 that extend from a crown 207,209 and 210 at a constant angle from the crown center, i.e., the rings212 are approximately zig-zag in shape, as opposed to sinusoidal forpattern 200, although in other embodiments a ring with curved struts iscontemplated. As such, in this embodiment a ring 212 height, which isthe longitudinal distance between adjacent crowns 207 and 209/210 may bederived from the lengths of the two struts 230 connecting at the crownand a crown angle θ. In some embodiments the angle θ at different crownswill vary, depending on whether a link 234 is connected to a free orunconnected crown, W-crown or Y-crown.

The zig-zag variation of the rings 212 occurs primarily about thecircumference of the scaffold (i.e., along direction B-B in FIG. 4). Thestruts 212 centroidal axes lie primarily at about the same radialdistance from the scaffold's longitudinal axis. Ideally, substantiallyall relative movement among struts forming rings also occurs axially,but not radially, during crimping and deployment. Although, as explainedin greater detail, below, polymer scaffolds often times do not deform inthis manner due to misalignments and/or uneven radial loads beingapplied.

The rings 212 are capable of being collapsed to a smaller diameterduring crimping and expanded to a larger diameter during deployment in avessel. According to one aspect of the disclosure, the pre-crimpdiameter (e.g., the diameter of the axially and radially expanded tubefrom which the scaffold is cut) is always greater than a maximumexpanded scaffold diameter that the delivery balloon can, or is capableof producing when inflated. According to one embodiment, a pre-crimpdiameter is greater than the scaffold expanded diameter, even when thedelivery balloon is hyper-inflated, or inflated beyond its maximum usediameter for the balloon-catheter.

Pattern 200 includes four links 237 (two at each end, only one end shownin FIG. 4) having structure formed to receive a radiopaque material ineach of a pair of transversely-spaced holes formed by the link 237.These links are constructed in such a manner as to avoid interferingwith the folding of struts over the link during crimping, which, asexplained in greater detail below, is necessary for a scaffold capableof being crimped to a diameter of about at most Dmin or for a scaffoldthat when crimped has virtually no space available for a radiopaquemarker-holding structure.

FIG. 6 depicts aspects of the repeating pattern of closed cell elementsassociated with pattern 200. FIG. 6 shows the portion of pattern 200bounded by the phantom box VB. Therein is shown cell 204. The verticalaxis reference is indicated by the axis B-B and the longitudinal axisA-A. There are four cells 204 formed by each pair of rings 212 inpattern 200, e.g., four cells 204 are formed by rings 212 b and 212 cand the links 234 connecting this ring pair, another four cells 204 areformed by rings 212 a and 212 b and the links connecting this ring pair,etc.

Referring to FIG. 6, the space 236 of cell 204 is bounded by thelongitudinally spaced rings 212 b and 212 c portions shown, and thecircumferentially spaced and parallel links 234 a and 234 c connectingrings 212 b and 212 c. Links 234 b and 234 d connect the cell 204 to theright and left adjacent rings in FIG. 4, respectively. Link 234 bconnects to cell 204 at a W-crown 209. Link 234 d connects to cell 204at a Y-crown 210. A “Y-crown” refers to a crown where the angleextending between a strut 230 and the link 234 d at the crown 310 is anobtuse angle (greater than 90 degrees). A “W-crown” refers to a crownwhere the angle extending between a strut 230 and the link 234 at thecrown 209 is an acute angle (less than 90 degrees). There is only onefree crown between each Y-crown and W-crown for the cell 204.

Additional aspects of the cell 204 of FIG. 5B include angles for therespective crowns 207, 209 and 210. Those angles, which are in generalnot equal to each other (see e.g., FIG. 7 for the “V59” embodiment of ascaffold having the pattern 200), are identified in FIG. 6 as angles267, 269 and 2680, respectively associated with crowns 207, 209 and 210.For the scaffold having the pattern 200 the struts 230 have strut widths261 and strut lengths 266, the crowns 207, 209, 210 have crown widths270, and the links 234 have link widths 261. Each of the rings 212 has aring height 265. The radii at the crowns are, in general, not equal toeach other. The radii of the crowns are identified in FIG. 6 as innerradii 262 and outer radii 263. Cell 204 may be thought of as a W closedcell element. The space 236 bounded by the cell 204 resembles the letter“W”.

The W cell 204 in FIG. 6 is symmetric about the axes B-B and A-A. The Wcell 204 is characterized as having no more than one crown 207 betweenlinks 234. Thus, a Y-crown crown or W-crown is always between each crown207 for each closed cell of pattern 200. In this sense, pattern 200 maybe understood as having repeating closed cell patterns, each having nomore than one crown that is not supported by a link 234.

A scaffold according to pattern 200 is stiffer than a similarlyconstructed scaffold having fewer connecting links. The scaffoldaccording to pattern 200 will be stiffer both axially and inlongitudinal bending, since there are more links 236 used. Increasedstiffness may not, however, be desirable. Greater stiffness can producegreater crack formation over a less stiff scaffold. For example, thestiffness added by the additional links can induce more stress on ringsinterconnected by the additional links 234, especially when the scaffoldis subjected to a combined bending (rings moving relative to each other)and radial compression and/or pinching (crushing). The presence of thelink 234 introduces an additional load path into a ring, in addition tomaking the ring stiffer.

Dimensions according to one embodiment of a scaffold having the W cellillustrated in FIG. 6 are shown in the Table of FIG. 7. These propertiesof the PLLA scaffold include a W cell having a reduced radii type ofcrown formation. The radius r_(b) is about 0.00025 inches, whichcorresponds to the smallest radius that could be formed by the laser.The 0.00025 inch radius is not contemplated as a target radius or limiton the radius size, although it has produced the desired result for thisembodiment. Rather, it is contemplated that the radius may be as closeto zero as possible to achieve a reduced profile size. The radius,therefore, in the embodiments can be about 0.00025 (depending on thecutting tool), greater than this radius, or less than this radius topractice the invention in accordance with the disclosure, as will beappreciated by one of ordinary skill in the art. For instance, it iscontemplated that the radii may be selected to reduce down the crimpedsize as desired. An inner radius at about zero, for purposes of thedisclosure, means the minimum radius possible for the tool that formsthe crown structure. An inner radius in accordance with some embodimentsmeans the radius that allows the distance S to reduce to about zero,i.e., struts are adjacent and/or touch each other when the scaffold iscrimped.

A scaffold according to FIGS. 4-6 exhibits a high degree of crushrecoverability, which is a desired attribute for aperipherally-implanted scaffold. The scaffold has a greater than about90% crush recoverability when crushed to about 33% of its startingdiameter, and a greater than about 80% crush recoverability when crushedto about 50% of its starting diameter following an incidental crushingevent (e.g., less than one minute); and/or greater than about 90% crushrecoverability when crushed to about 25% of its starting diameter, and agreater than about 80% crush recoverability when crushed to about 50% ofits starting diameter for longer duration crush periods (e.g., betweenabout 1 minute and five minutes, or longer than about 5 minutes). Otherattributes of a scaffold suited for use a peripheral scaffold are acrown angle of between 105 and 95 degrees, or less than 115 degrees.

The above description of illustrated embodiments of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific embodiments of, and examples for, the invention aredescribed herein for illustrative purposes, various modifications arepossible within the scope of the invention, as those skilled in therelevant art will recognize.

These modifications can be made to the invention in light of the abovedetailed description. The terms used in the claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification. Rather, the scope of the invention is to bedetermined entirely by the claims, which are to be construed inaccordance with established doctrines of claim interpretation.

What is claimed is:
 1. A method of crimping, comprising the steps of:providing a scaffold formed from an expanded tube comprising a polymer,the scaffold having a network of interconnected closed cells; crimpingthe scaffold to a balloon catheter, the scaffold having a pre-crimpdiameter and a final crimp diameter wherein the ratio of the pre-crimpdiameter to final crimp diameter is at least 3:1, including the steps ofreducing the scaffold diameter from the pre-crimp diameter to anintermediate diameter that is less than about 30% of the pre-crimpdiameter, after reducing the scaffold to the intermediate diameter,reducing the diameter down to the final crimp diameter and pressurizingthe balloon, and initiating balloon pressurization only after thescaffold has attained the intermediate crimp diameter and prior to thescaffold attaining the final crimp diameter.
 2. The method of claim 1,further including the step of reducing the scaffold to a firstintermediate diameter, removing the scaffold from a crimp mechanism,returning the scaffold to the crimp mechanism, then reducing thediameter to a second intermediate diameter that is about 30% of thepre-crimp diameter.
 3. The method of claim 1, wherein balloon pressureis applied while the scaffold diameter is being reduced from theintermediate diameter to the final crimp diameter.
 4. The method ofclaim 1, wherein the ratio of the pre-crimp diameter to final crimpeddiameter is at least 4:1.
 5. The method of claim 1, wherein the scaffoldhas no more than 4 link elements interconnecting adjacent rings of thescaffold.
 6. The method of claim 5, wherein the link elements extendparallel to a longitudinal axis of the scaffold.
 7. The method of claim1, wherein the balloon pressure is about 50 psi.
 8. The method of claim1, wherein the scaffold comprises PLLA.
 9. The method of claim 1,wherein the polymer is characterized by a glass transition temperaturerange having a lower limit of TG-low and crimping is done at atemperature of between about 5 and 15 degrees below TG-low.
 10. A methodof crimping, comprising the steps of: providing a scaffold comprising apolymer and a balloon, wherein the scaffold and balloon satisfy theinequality1.1×(SDi)×(1.2)⁻¹ ≦SDpc≦1.7×(SDi)×(1.2)⁻¹, where SDpc is a scaffoldpre-crimp diameter, and SDi is a balloon nominal inflation diameter; andcrimping the scaffold to the balloon, including the steps of reducingthe scaffold diameter from the pre-crimp diameter to an intermediatediameter that is less than about 30% of the pre-crimp diameter, dwellingthe scaffold at the intermediate diameter to allow for stress relaxationprior to reducing the diameter to a final crimp diameter, and afterdwelling the scaffold at the intermediate diameter, inflating theballoon and reducing the diameter to the final crimp diameter.
 11. Themethod of claim 10, wherein the polymer is characterized by a glasstransition temperature range having a lower limit of TG-low and crimpingis done at a temperature of between about 5 and 15 degrees below TG-low.12. The method of claim 11, wherein the reducing the scaffold diameterfrom the pre-crimp to intermediate diameter comprises a plurality ofdwell periods prior to reducing the diameter to the intermediatediameter.
 13. The method of claim 11, wherein the scaffold is made froma radially expanded tube comprising PLLA.
 14. The method of claim 10,wherein the dwell period is 30 seconds.
 15. The method of claim 10,wherein the balloon is inflated before the scaffold diameter is reducedto the final crimp diameter.
 16. The method of claim 10, wherein a ratioof pre-crimp to fully crimp diameters is between 2.5:1 and 3:1 and theballoon nominal inflation diameter is 6 mm.